The parasites traveling each of these cycles had been designated as individual species, but no one actually knew whether they weren’t actually a single species scattered among different regions and hosts. Sukhdeo got hold of Trichinella from Russia, from Canada, and from Africa, as he was told, and he ground up each sample and infected mice with them. He extracted the antibodies that the mice’s immune systems produced against the ground-up parasites and compared them to judge how similar they were to one another.
Eventually he stopped to wonder why he was doing what he was doing. His experiments were based on the assumption that individuals of a species look similar to one another. This is usually a pretty reliable assumption, but biologists have recognized that it’s not always the case. Poodles and Dobermans belong to the same species, for instance. On the other hand, two beetles that look practically identical may belong to separate species. Rather than focus on appearances, biologists these days define a species as a group of organisms that breed together and don’t breed with other groups. It’s out of that isolation that evolution then makes a species distinct from others.
Sukhdeo decided that the best way to study the species of his parasites was to work out their sex life. He dissected Trichinella cysts out of muscle and teased out the worms, only 250 microns long. He’d check their sex and then get the parasite into a syringe, which he’d inject into the stomach of a mouse. Then he’d go back to his cysts and find a parasite of the opposite sex, and then inject it into the mouse’s stomach as well. A month later he’d look at the mouse’s muscle to see whether they had mated and produced young.
Sukhdeo concluded that the African form was probably a subspecies and not a separate species of its own. But the experiment actually raised a much deeper, much more interesting question. How did the parasites find each other?
Apply the Fantastic Voyage method: It would be as if you were thrown down into a dark cavernous tunnel twelve miles long, lined on all sides with slippery, tightly packed, man-sized mushrooms. If you were set down randomly in there and moved around randomly, there’d be no hope of finding someone else in such a place. And yet, Trichinella—without a map or even much of a brain—always did.
Sukhdeo wanted to know how they did it, but his adviser told him not to try. “‘You can’t find out how these animals go wherever they go because for a hundred years parasitologists have been trying to find out the answer and they haven’t been able to. Better people than you have tried.’”
Sukhdeo ignored the advice and set out to find the secret to parasite navigation. Unfortunately, he set out in the wrong direction. He assumed that like animals on the outside, parasites must follow a gradient. A shark smells the blood of a wounded seal from miles away and heads for it, thanks not only to its sharp nose but to the simple law of how blood spreads in water. The farther away the blood travels from the seal, the thinner it gets. If a shark keeps heading along a rising gradient, it will automatically reach the source. As soon as it veers away in the wrong direction, the blood trails off, and it can right itself. Gradients work in the air just as well as in the water. They help lead bees to flowers and hyenas to carcasses. Tracking gradients works so well at sea and on land that it only made sense that parasites must use them as well. Parasitologists searched for the scent of a gallbladder, the whiff of an eye. They didn’t find any.
For years, Sukhdeo tried to find the secret for himself. He built chambers out of Plexiglas in which he could put a parasite, and then he’d add different chemicals to see if it would swim toward it. At first he kept his entire lab heated to body temperature. Then he invented a system of tubes to circulate warm water around his artificial gut. “I would try to sample everything they encountered in the host. First I tried salivary secretions, and then I would move down the gut.” Nothing he did made sense. He couldn’t get the parasites to swim toward or away from any substance he put in the chamber.
They did react sometimes, but in a way that made no sense at all. “Whenever these little parasites encountered bile they started moving like crazy,” Sukhdeo said. “That wasn’t what I wanted—I wanted something that attracted them. Initially they would move back and forth fifty times a minute, and if you put bile in, there was an instantaneous change and they started moving sinusoidally.”
Sukhdeo kept looking for the key to parasite navigation after he moved to the University of Toronto. As he searched he drifted into an academic limbo. At Toronto he met his wife, Suzanne, who was also getting her Ph.D. in parasitology with the director of his lab. When the director developed Alzheimer’s disease, Sukhdeo took over the lab and became Suzanne’s dissertation advisor. If he had wanted to have a real career in parasitology, he should have been looking for jobs elsewhere, but instead he lingered in Toronto, applying for more money each year to carry on his experiments. For six years he floated in this dead-end existence, but he found that it gave him the freedom to search for answers that other scientists thought were unreachable. “I had nothing to lose,” Sukhdeo says. “I could do anything I wanted, and I had no future.”
He decided to extend his research to other species, such as the liver fluke, Fasciola hepatica. A relative of the blood fluke, it has a similar life cycle. It lives inside cows and other grazing mammals, and its eggs pass out of its host’s body with feces. It hatches from its egg and swims in search of a snail, where a couple of generations grow up. Cercariae emerge from the snail and swim away from the snail until they hit any object—usually a rock or a plant—and build themselves a tough transparent cyst. When another grazing mammal eats them, their acid-proof shell carries them safely through the stomach and into the intestines. Once in the intestines, they break loose and burrow out into the abdominal cavity and then head for the liver. There they grow into adults—leaf-shaped inch-long animals that can cram into a liver by the hundreds and live for eleven years. Liver flukes can sometimes get into humans, but the real danger they pose is to livestock. In tropical countries, between 30 and 90 percent of cattle carry them, and they cause $2 billion in damage every year. Yet, despite the massive harm they cause and despite decades of research, scientists had no idea how they managed to find the liver.
Sukhdeo built himself new chambers out of brass and aluminum and put liver flukes into them. He spent three years trying out different compounds given off by the liver—chemicals that might lure the flukes to their final home. Out of sheer exasperation, he tracked down a prominent liver physiologist to see if there was some attractant he had overlooked.
“He thought about it for a long time and said, ‘You know, son, around the liver there is a capsule; it’s called Glisson’s capsule?’”